1. Field of the Invention
Examples of the subject matter disclosed herein generally relate to methods and systems for indicating the presence of gas hydrate and shallow gas which includes the use of seismic data, in particular methods for indicating that include the use of AVO responses in seismic data.
2. Discussion of Background
Although seismic amplitude is affected by many factors, AVO techniques have shown to be useful for direct hydrocarbon indication over the past three decades, especially for gas sand reservoir in the clastic depositional settings. In contrast, the application to identify shallow gas and gas hydrate has not reached the same level. Gas hydrates refer to naturally occurring solid composed of crystallized water (ice) molecules containing molecules of natural gas, which may be mainly methane and higher order hydrocarbons. When the term hydrate is used in the context of this application, gas hydrates are meant. Ecker et al. (1998) use rock physics-based AVO modeling to investigate the internal structure of hydrate-bearing sediments offshore Florida. Carcione and Tinivella (2000) compute AVO curves for consolidated Berea sandstone with gas hydrate and free gas. Furthermore, a few studies attempt to separate hydrate-bearing sediments (gas hydrate without free gas below) from hydrate-over-gas sediments (gas hydrate overlies free gas) from seismic amplitude; both are drilling hazards but different drilling strategies and protocols should be applied. The present invention provides a novel AVO modeling technique found to robustly and reliably perform the differentiation.
In Shuey's two-term approximation to the Zoeppritz equations, the P-wave reflection coefficient can be approximately written as a function with two parameters: AVO intercept (A) and AVO gradient (B) (Shuey, 1985). In general, deepwater sediments follow normal compaction processes, which define background trends. AVO anomalies can be observed from the crossplot of the intercept and gradient because they deviate from the background trend. Rutherford and Williams divided AVO anomalies into three categories (Classes I, II and III) based on normal incidence reflection coefficient (1989). Castagna and others proposed an additional category, Class IV, and presented the anomalies in terms of locations in the crossplot (Castagna et al., 1997 and Castagna and Swan, 1998). In this study, we investigate the AVO anomalies related to gas hydrate and/or free gas by simulating their AVO reflection coefficient responses and intercept and gradient in the A-B plane.
The AVO classification presented by Rutherford and Williams (1989) and Castagna and Swan (1997) has become the industry standard for AVO analysis for oil and gas exploration. They classify gas sand responses into four classes. Class I is high impedance sand underlying low impedance shale and has a positive intercept and negative gradient for top of gas sand. Class II is characterized by a small impedance contrast for which the impedance of the sand is about the same as the overlying shale, the gradient is negative and the intercept may be negative or positive. Class III has a low impedance sand underlying high impedance shale characterized by increasing amplitude with offset; Class IV also has low impedance sand underlying high impedance shale but exhibits amplitude decreasing with offset. Thus the intercepts are negative for both Class III and Class IV whereas the gradients are negative for Class III and positive for Class IV.